A main soft segment part of polyurethane resins which have been conventionally produced on an industrial scale, namely a polyol, is classified into a polyether type represented by polypropylene glycol and polytetramethylene glycol, a polyester polyol type represented by dicarboxylic acid based polyesters, a polylactone type represented by polycaprolactone, and a polycarbonate type obtained by reacting a carbonate source and a diol (Non-Patent Document 1).
Of these, though the polyether type is excellent in hydrolysis resistance, flexibility, and elasticity, it is considered to be inferior in mechanical strength such as abrasion resistance, flexibility resistance, etc., heat resistance, and weather resistance. On the other hand, though the conventional polyester type is improved in heat resistance and weather resistance, it cannot be used depending upon an application because hydrolysis resistance of an ester segment thereof is low. Though the polylactone type is considered to be a slightly better in hydrolysis resistance as compared with adipates, it is unable to completely suppress the hydrolysis because it similarly has an ester group.
Furthermore, though the polycarbonate type is excellent in hydrolysis resistance and durability, it involves such a drawback that handling operability is poor because a solution viscosity of a polyol itself or a polyurethane produced using this as a raw material is high. In addition, though it is also proposed to use these polyester type, polyether type, polylactone type, and polycarbonate type upon being mixed and copolymerized, the respective drawbacks cannot be completely compensated yet.
In addition, in recent years, global-scale awareness concerning environmental issues is increasing, and raw materials derived from biomass resources such as plants, etc. but not petroleum-derived raw materials affecting the warming of the earth are expected. However, almost all of the foregoing polyols are derived from petroleum exclusive of an extremely part of raw materials.
Furthermore, in polyester polyols which are most widely used at present, polyester polyols synthesized from adipic acid are leading. However, in producing adipic acid, a nitric acid oxidation method is adopted, and there is present an environmental issue that N2O which is conspicuously large in a warming effect following the production as compared with CO2.
Then, in order to solving these problems, polyester polyols having a variety of structures are proposed. For example, there is a method for forming a polyester polyol by mixing and copolymerizing succinic acid as a dicarboxylic acid other than adipic acid and a diol without adopting nitric acid oxidation for the production thereof, specially a method of co-mixing succinic acid and an oligomer of ethylene glycol (Patent Document 1).
But, though polyester polyols using petroleum-derived succinic acid and polyurethanes produced therefrom are known and industrially produced, the polyester polyols using succinic acid as a raw material are generally poor in handling properties.
For example, the polyester polyols using succinic acid involved such problems that reaction control in the polyurethane reaction is difficult; the molecular weight of a polyurethane resin is liable to increase; in the case of using a polyisocyanate with low reactivity, etc. as a raw material, the polyurethane reaction becomes instable; and the like. In addition, as compared with those produced using generally widely used adipic acid as a raw material, the resulting polyurethane resins have such properties that as a physical property of the resin, are high in hardness and high in elastic modulus of tensile strength, and their applications for use were restricted.
On the other hand, from the viewpoint of protecting the global environment in the recent years, polyurethane resins derived from biomass resources are demanded. However, sebacic acid and castor oil are merely used in small amounts for limited applications, and polyurethane raw materials derived from biomass resources have been expected.
In the recent years, there is disclosed a technology for producing a polyester polyol using, as a raw material, biosuccinic acid obtained by the fermentation method (Patent Document 2). However, a polyurethane using the polyester polyol produced by the technology described in Patent Document 2 merely exhibits mechanical characteristics equal to those in the case of using a polyester polyol produced using, as a raw material, succinic acid derived from petroleum resources.
In the light of the above, according to any of the foregoing methods, the resulting polyester polyols are not ones having a balance of physical properties of the polyester polyol per se, good handling properties, color, and a balance of easiness of reaction control or mechanical physical properties when formed into a polyurethane. Thus, the development thereof has been expected.